语法功能：  表示与谓语动词同步发生w.w.w.k.s.5.u.c.o.m

2) 完成形式：He pretended not to have seen me.

被动形式：The book is said to have been translated into many languages.

语法功能：表示发生在谓语动词之前

二）不定式常考的考点：

1）不定式做定语----将要发生

2）不定式做状语----目的

3）不定式充当名词功能---To see is to believe.

三)不定式的省略

1)感官动词 see, watch, observe, notice, look at, hear, listen to, smell, taste, feel

+ do　 表示动作的完整性，真实性；

+ doing 表示动作的连续性，进行性

I saw him work in the garden yesterday.

昨天我看见他在花园里干活了。(强调"我看见了"这个事实)

I saw him working in the garden yesterday.

昨天我见他正在花园里干活。（强调"我见他正干活"这个动作）

v  感官动词后面接形容词而不是副词：The cake tastes good; It feels comfortable.

 2) 使役动词have bid make let 等词后不定式要省略但同1)一样被动以后要还原to

I’d like to have John do it.

I have my package weighed.

Paul doesn’t have to be made to learn.

 3) help help sb do  help sb to do help do  help to do

四)有些动词后只跟不定式如：

want，wish，hope，manage，promise，refuse，pretend，plan, offer，decide，agree，expect, allow sb to do,  cause sb to do, permit sb to do, enable sb to do, force sb to do, be more likely to do, love to do, warn sb to do, be able to do, be ambitious to do, begin to do, start to do

We agreed _______ here，but so far she hasn't turned up yet．（NMET 1995）

A．having met    B．meeting    C．to meet    D．to have met （Key：C）

 X=3m>L  故A掉下木板。

当时，解得 

（4）设B的水平部分长度为x时，A不会掉下

    不合理，舍去。

由   得

A从最高点返回P点的过程中产生的内能

当时，

    解得：  

（3）设A返回P点时速度为